Engines may increase output power by using boosting devices that compress intake air. Since charge compression increases air temperature, charge air coolers may be utilized downstream of a compressor to cool the compressed air, further increasing the potential power output of the engine. As intake air passes through the charge air cooler and is cooled below a dew point, condensation occurs. The condensate may be accumulated at a trap and delivered to the running engine subsequently, at a controlled rate. The introduction of water into the engine, however, can increase the likelihood of misfire events. Engine control systems may have to employ various knock and misfire control approaches to address the knock.
One example approach for addressing humidity induced combustion issues is shown by Sasaki et al. in US 2011/0303187. Therein, a knock-limit ignition timing is adjusted based on deviations in a fuel octane content from a basic fuel octane content as well deviations in ambient humidity from a basic ambient humidity. This allows misfire events arising due to a sudden change in fuel octane content and high ambient humidity to be reduced.
However the inventors herein have identified potential issues with such an approach. Even with the adjusted ignition timing, misfire events may occur. Specifically, condensate formation may involve various factors including, but not limited to, ambient humidity. Other factors that may affect condensate formation at the charge air cooler include, for example, mass air flow, ambient temperature, charge air cooler outlet temperature, ambient temperature, charge air cooler to ambient pressure ratio, EGR, etc. In addition, condensate formation and condensate purging may affect the intake manifold humidity. Thus, there may be conditions when ambient humidity is low but intake manifold humidity is high. If spark ignition timing is adjusted based on ambient humidity during those conditions, the ingested condensate can slow the burn rate of combustion and degrade combustion efficiency. Likewise, there may be conditions when ambient humidity is high but intake manifold humidity is low. If spark timing is adjusted based on ambient humidity during those conditions, combustion efficiency may again be reduced and a frequency of knock events may increase.
In one example, some of the above issues may be addressed by a method for a boosted engine comprising adjusting spark timing from an initial setting based on ambient humidity to a final setting, the adjusting based on a change in condensate level at a charge air cooler. In this way, knock and misfire can be reduced when condensate is stored or purged at the cooler.
As one example, during conditions when condensate is being stored at a charge air cooler, borderline knock limits and spark timing may be retarded. As such, due to condensate being stored at the charge air cooler, a resulting intake manifold humidity may be lower than the ambient humidity. The amount of spark retard applied may be based on a difference between the resulting intake manifold humidity and the ambient humidity. By applying the spark retard, the likelihood of knock occurring during the storing can be reduced and combustion stability can be improved. As another example, during conditions when condensate is being released from the charge air cooler to the engine intake, borderline knock limits and spark timing may be advanced (or retarded less). As such, due to condensate being released from the charge air cooler, a resulting intake manifold humidity may be higher than the ambient humidity. The amount of spark advance applied may be based on a difference between the resulting intake manifold humidity and the ambient humidity. By applying the spark advance, knock toleration is and combustion stability is improved during the purging. In comparison, during steady state conditions at the charge air cooler, when there is no substantial change in condensate levels, spark timing may be maintained.
In this way, spark adjustments may be performed based on changes to intake manifold humidity resulting from storage or release of condensate at or from a charge air cooler. By advancing borderline knock limits and spark timing when condensate is purged, the increased intake manifold humidity from the purging can be advantageously used to limit knock. By retarding borderline knock limits and spark timing when condensate is being stored, combustion stability can be improved. Overall, condensate management is accomplished without degrading engine performance.
It will be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description, which follows. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined by the claims that follow the detailed description. Further, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.